The present invention relates to a rocker arm made of sheet metal which is manufactured by pressing from a metal plate, out of rocker arms incorporated in a valve driving mechanism of an engine for converting rotation of a cam shaft to a reciprocating motion of a valve unit (including a suction valve and an exhaust valve), as well as an improvement in a manufacturing method thereof.
The present invention also relates to a cam follower provided with a sheet metal rocker arm and an improvement in an assembling method thereof.
A reciprocating engine (reciprocating piston engine) is provided with a suction valve and an exhaust valve which opens and closes in synchronism with a rotation of a crank shaft, except a two-cycle engine provided in certain types. In such a reciprocating engine, a motion of a cam shaft which rotates in synchronism with a rotation of the crank shaft (at a rotation speed of ½ in case of a four-cycle engine) is transmitted to the intake valve and the exhaust valve by use of a rocker arm, and the intake valve and the exhaust valve reciprocate along the axial direction.
Conventionally, such a rocker arm incorporated in the valve driving mechanism of the engine is generally formed by casting (as a cast iron or aluminum die cast product). However, a cast product is too heavy (in case of an iron cast) or bulky (in case of an aluminum die cast) for maintaining a sufficient strength. In addition, since the rocker arm is generally manufactured by a lost wax method, the manufacturing cost is unavoidably increased. For this reason, it is recently considered to manufacture such rocker arm by pressing from a metal plate such as a steel plate, which is partially realized.
A conventional manufacturing method of a sheet metal rocker arm considering such circumstances is disclosed in, for example, Japanese Patent Application Laid-Open No. 3-172506. FIGS. 19 to 22 show the manufacturing method of a sheet metal rocker arm disclosed in this application. According to this conventional method, first a metal plate (such as a carbon steel plate having a thickness of 2 to 4 mm) as a blank is punched by pressing, so as to form a blank 1 having a shape as shown in FIG. 19A and a thickness of t1, as shown in FIG. 19B.
Next, this blank 1 is subjected to bending by pressing to form a first intermediate blank 2 as shown in FIGS. 20A and 20B. This first intermediate material 2 comprises a pair of side walls 3, 3 parallel to each other, a connecting portion 4 for connecting the edges of both side walls 3, 3 in the width direction, a roller receiving recess 5 formed in a middle part of a space surrounded in three directions by the both side walls 3, 3, and the connecting portion 4, and a pivot portion 6 as a spherical concave surface formed in a middle part at a position nearer one end of the connecting portion 4.
Subsequently, a through hole 7 which has a Japanese hand drum shape when seen from the side parallel to the side walls 3, 3, as shown in FIGS. 21A and 21B, is formed in a portion which is a part of the connecting portion 4 for forming the first intermediate blank 2 as described above and is corresponding to the roller receiving recess 5, as a second intermediate blank 8. Arched protrusions 9, 9 which are part of the second intermediate blank 8 are provided to sandwich this through hole 7 from both sides thereof in the width direction (the up-and-down direction in FIG. 21A) in a state that the protruding edges of both of the protrusions 9, 9 are placed opposite to each other. The through hole 7 has a narrower width W7 at the center thereof, compared with the width at a part nearer either of the ends thereof.
When a part surrounding the though hole 7 of the second intermediate blank 8 as described above is subjected to burring and ironing, a third intermediate blank 10 as shown in FIG. 22 is obtained. In this third intermediate blank 10, the through hole 7 becomes an opening 11 which has a rectangular shape when seen from the side parallel to the side walls 3, 3, and the shape of the other end portion of the connecting portion 4 is adjusted to become a valve engagement portion 12 for abutting on the base end portion of the valve unit constituting the suction valve or the exhaust valve. At the subsequent step, in the third intermediate blank 10 as described above, circular holes for supporting both ends of a support shaft for supporting to allow free rotation a roller which is engaged with the cam are formed at positions aligned with each other on both of the side walls 3, 3, thereby completing a sheet metal rocker arm. Then, when such sheet metal rocker arm is assembled in the engine, the outer peripheral surface of the roller which is supported by the roller receiving portion 5 in a rotating manner is brought into contact with the outer peripheral surface of the cam, the leading end portion of a lash adjuster is caused to abut upon the pivot portion 6, and the base end surface of the suction valve or the exhaust valve is caused to abut upon the valve engagement portion 12.
The thickness t3 of each of the laterally paired side walls 3, 3 for constituting the sheet metal rocker arm manufactured in the manner described above is substantially equal to the thickness t1 of the blank 1 (FIG. 19B) (t3≈1). The thickness t3 of each of the side walls 3, 3 and the thickness t4 of the connecting portion 4 including the pivot portion 6 and the valve engagement portion 12 (FIG. 22B) are also substantially equal to the thickness t1 of the blank 1 (t1≈t3≈t4).
More specifically, since formed of one metal plate in a unitary structure mainly by pressing, the conventional sheet metal rocker arm mentioned above has substantially a uniform thickness over the entire surface thereof except a part of the pivot portion 6 and a part followed by a part of the metal plate. Also, in case of a conventional technology other than Japanese Patent Application Laid-Open No. 3-172506 mentioned above, a sheet metal rocker arm which is formed of one metal plate in a unitary structure mainly by pressing has substantially a uniform thickness over the entire surface thereof.
On the other hand, there is conventionally known a structure of a rocker arm in which two or three members respectively formed by pressing of a metal plate are connected and fixed to each other by welding. In case of a sheet metal rocker arm which is formed by combining plural members as stated, the thickness of the connecting portion including the pivot portion and the valve engagement portion is formed greater than the thickness of each of the side walls.
According to the conventional technology described above, inconveniences as stated below will be brought about. First, according to the technology disclosed in Japanese Patent Application Laid-Open No. 3-172506 for forming a sheet metal rocker arm from one metal plate in a unitary structure, the thickness of the formed sheet metal rocker arm is uniform substantially over the entire surface thereof. On the other hand, when the rocker arm is in use, a stress acting on the connecting portion 4, especially that acting in the vicinity of the valve engagement portion 12, is greater, compared with that stress acting on another portion such as the side walls 3, 3. For this reason, when the thickness is uniform, the connecting portion 4, specially in the vicinity of the valve engagement portion 12, is disadvantageous in terms of the strength, compared with other portions, and the rigidity also may be lowered in some cases. In case of the conventional technology, the thickness of the metal plate for forming the sheet metal rocker arm is made to be large in order to secure a sufficient strength and rigidity of a portion in the vicinity of the valve engagement portion 12. Consequently, the thickness of the other portions such as the side walls 3, 3 is greater than that originally required, so that the size and the weight of the sheet metal rocker arm can not be sufficiently reduced. In addition, the cost of materials is increased.
In case of the sheet metal rocker arm in which two or three members respectively formed of a metal plate by pressing are connected and fixed to each other by welding, the thickness of the connecting portion including the valve engagement portion can be made greater than the thickness of another portion such as the side wall. On the other hand, however, after plural members are formed separately, these members are required to be combined with each other and bonded together by welding. Consequently, the number of processing steps increases and extra labor is required for controlling the constituent parts. Since complicated and precise equipment is required for positioning the respective members when they are assembled, it is unavoidable to increase the cost, as well as to increase the number of processing steps and to require extra labor for controlling the parts. Moreover, the quality of the obtained sheet metal rocker arm (precision) is often inferior to that of the rocker arm formed in a unitary structure.
Though having a superior toughness to the cast-type rocker arm, the sheet metal rocker arm may be elastically deformed more easily depending on the direction of action of the force. That is, since each of the paired walls 3, 3 for bridging both ends of a pivot for supporting the roller takes a flat-plate shape, if a force in a right-angled direction is applied on the side walls 3, 3, the side walls 3, 3 are elastically deformed comparatively easily. On the other hand, when both ends of the pivot are plastically deformed toward the inner peripheral surfaces of both of the through holes for connecting and fixing both ends of the pivot to each other, a force is applied onto portions which are provided on the side walls 3, 3 with the through holes formed thereon in a direction in which both portions come toward each other. Then, the side walls 3, 3 are elastically deformed on the basis of this force.
In case of the conventional sheet metal rocker arm, the paired side walls 3, 3 are formed to be parallel to each other prior to when both ends of the pivot are plastically deformed. For this reason, when both ends of the pivot are plastically deformed toward the inner peripheral surfaces of the through holes, the paired side walls 3, 3 are formed to be non-parallel to each other. Accordingly, the inner side surfaces (the side surfaces opposite to each other) of the side walls 3, 3 and both end surfaces in the axial direction of the roller supported in a middle part of the pivot in a rotating manner are formed to be non-parallel to each other. As a result, a so-called edge abutment is brought about in which the inner side surfaces of the side walls 3, 3 and both end surfaces of the roller in the axial direction are not brought into contact with each other in a uniformly wide area, but may be brought into contact with each other in a very narrow area, or the edges of the side walls 3, 3 and both end surfaces of the roller in the axial direction are brought into contact with each other.
In such a state, it is difficult to satisfactorily form an oil film between the inner side surfaces of the side walls 3, 3 and the respective both end surfaces of the roller in the axial direction for decreasing friction between these both surfaces. This is not preferable since a resistance required for a rotation of the roller may be increased, or an amount of abrasion of the roller or the sheet metal rocker arm may be increased.
When the cam follower with the sheet metal rocker arm is in use, the roller is rotated inside the roller receiving recess 5, which is provided on this sheet metal rocker arm. When this roller is displaced in the axial direction with respect to the pivot which is supported on and fixed to the sheet metal rocker arm, the end surface of the roller in the axial direction and the inner side surface of one of the side walls 3 rub against each other. Accordingly, it is required to decrease a frictional resistance of a contact portion between these end surfaces of the roller in the axial direction and the inner side surfaces of the side walls 3, 3 for reducing a rotational resistance of the roller and for reducing abrasion of this roller and the sheet metal rocker arm.
However, in case of the cam follower provided with the conventional sheet metal rocker arm, such requirements are not always taken into consideration.